It is well known that contact lenses can be used to improve vision. Various contact lenses have been commercially produced for many years. Hydrogel contact lenses are very popular today. These lenses are formed from hydrophilic polymers and copolymers containing repeating units from hydroxyethylmethylacrylate (HEMA). Of these contact lenses formed from copolymers of HEMA and methacrylic acid, are among the most comfortable, and have the lowest rate of adverse events. Contact lenses formed from copolymers of HEMA and MAA, such ACUVUE contact lenses, display substantial amounts of lysozyme uptake (greater than 500 μg) and retain a majority of the uptaken proteins in their native state. However, hydrogel contact lenses generally have oxygen permeabilities that are less than about 30.
Contact lenses made from silicone hydrogels have been disclosed. These silicone hydrogel lenses have oxygen permeabilities greater than about 60, and many provide reduced levels of hypoxia compared to conventional hydrogel contact lenses. However, silicone hydrogel lenses have different rates for adverse events than conventional hydrogels, and it would be desirable to maintain the oxygen transmissibility of a silicone hydrogel, but achieve the low adverse event rate of the best conventional hydrogel lenses. Unfortunately, attempts to add anionic components to silicone hydrogels in the past have produced contact lenses which are not hydrolytically stable and display moduli which increase when exposed to water and heat. For example, the modulus of Purevision lenses (Bausch & Lomb) increase from 155 psi to 576 psi when heated at 95° C. for one week. It is believed that the cause of this increase in modulus is the hydrolysis of terminal siloxane groups followed by condensation reactions to form new siloxane bonds and introduce new crosslinks. Even though Purevision lenses contain about 1 weight % ionicity, they uptake relatively low lysozyme levels (less than about 50 μg), and a majority of the protein uptaken is denatured.
It has been suggested that the instability of ionic silicone hydrogels could be reduced by using silicones components having bulky alkyl or aryl groups instead of silicone monomers such as 3-methacryloxypropyltris(trimethylsiloxy)silane (“TRIS”) or 2-methyl-,2-hydroxy-3-[3-[1,3,3,3-tetramethyl-1-[(trimethylsilyl)oxy]disiloxanyl]propoxy]propyl ester (“SiGMA”). However, the bulky siloxane monomers are not commercially available and may be expensive to make.